1. Field of the Invention
This invention relates to an evaporative emission control system for an internal combustion engine, in which a vapor storage canister adsorbs fuel vapors generated in fuel reservoirs such as a float chamber in a carburetor and a fuel tank when the engine is stopped so that fuel vapors stored in the canister may be released into an intake passage of the engine during engine operation in order to prevent contamination of the atmosphere and save fuel.
2. Description of the Prior Art
In prior art the fuel vapors stored in such a canister have been purged, as controlled, into an intake passage of engine through a purge control valve which is operated by an intake vacuum pressure in accordance with engine operating conditions. However, the quantity of fuel vapors adsorbed by or desorbed from the canister is indefinite. Therefore, fuel vapors released into the intake passage unstabilize an air-fuel ratio of mixture during operation of the engine. A mixture supplied to the engine, particularly during engine operation at low temperature, is required to stabilize its rich air-fuel ratio because this change of air-fuel ratio of the mixture due to the induction of fuel vapors brings about an incomplete combustion and an increase in amount of pollutants emitted from the engine.